G-protein-coupled receptors (GPCRs) are a large family of proteins that are involved in a wide range of functions (including various autocrine, paracrine and endocrine processes). GPCRs show considerable diversity at the sequence level and can be separated into distinct families on the basis of their sequence.
The family C GPCR receptors (which are also known as family 3 GPCRs) are generally composed of four elements: an N-terminal signal sequence, a large hydrophilic extracellular agonist-binding region containing several conserved cysteine residues which may be involved in disulphide bonds, a shorter region containing seven transmembrane domains, and a C-terminal cytoplasmic domain of variable length (see, e.g., Brauner-Osborne, Curr. Drug Targets 2007 8: 169-84). Family C GPCR members include the metabotropic glutamate receptors, the extracellular calcium-sensing receptors, the gamma-amino-butyric acid (GABA) type B receptors, and the vomeronasal type-2 receptors, for example (see, e.g., Tanabe Neuron 1992 8: 169-79; Brown, Nature 1993 366: 575-80; Sullivan, J. Pharmacol. Exp. Ther. 2000 293: 460-7; and Ryba, Neuron 1997 19: 371-9).
As family C GPCRs are involved in many important physiological processes, they are promising targets for drug development.